The customization comes in your choice of extras and then a special fill-in section. This is where you can write any specific instruction you might have. Like easy on the sour cream. Read more
The kid's meal burrito is almost as big as a regular burrito. $4 for it with lemonadeslushy. About 1/2 the price of a comparable adult meal. Also, get the pulled pork not steak.
Basically like WhichWich, write your order on a bag style. Ok food, but since it's the bag order method, you can't customize like you do at Chipotle. Also, the steak and salsa are tastier at there.